1. Field of the Invention
The present invention relates to organic light emitting display (OLED) devices, and in particular relates to RGBW format OLED devices and methods for rendering images utilizing the same.
2. Description of the Related Art
Flat-panel displays, such as organic light emitting displays (OLED) and liquid crystal display (LCD), of various sizes are used in many computing and communication devices. In particular, OLED devices are used both indoors and outdoors under various ambient lighting conditions. Indoor applications have relatively low ambient illumination and require lower levels of display luminance. In contrast, outdoor applications can have a high ambient luminance and may require higher levels of display luminance together with low display reflectance.
FIG. 1 is a cross-section schematically illustrating a conventional white-emitting OLED structure. In FIG. 1, a substrate 10 with an indium tin oxide (ITO) anode 20 thereon is provided. A hole injection layer 22, a hole transporting layer 24, a yellow emitting layer 25, a blue emitting layer 26, and an electron transporting layer 28 are sequentially disposed on the ITO anode 20. A LiF/Al cathode 30 is disposed on the electron transporting layer 28. Emission from the blue emitting layer 26 and the yellow emitting layer 25 combine to provide a white emission that can be filtered appropriately to provide R, G, B and W emission from the display device.
While technology for OLED devices has improved dramatically, there are still some issues in its utilization in most potential applications, which are lifetime, power consumption, and others. Especially for mobile applications, low power consumption is required.
There are several methods to reduce power consumption of RGBW format OLED devices such as increasing the OLED material lighting efficiency or reducing the threshold voltage of an OLED device. Conventional OLED devices with color filter on array (COA) combined with RGBW driving function are typical methods for reducing OLED device power consumption.
FIG. 2 is a 1931 CIE x,y-chromaticity diagram showing the color gamut of a conventional RGBW format OLED device. The white point W of the unfiltered emitter (0.361, 0.380) and the desired white point D65 (0.3127, 0.3290) are provided in FIG. 2. Any color within each of the sub-triangles with the triangle RGB can be created by the appropriate combination of primary colors with white color. Since the unfiltered white point W deviates from the desired white point D65, an addition method is required to compensate for the unfiltered white pixel.
In practice, the color coordinate of a white pixel of an OLED device is not always on the desired white point D65 due to the characteristics of the OLED material and manufacturing recipes. Therefore, when displaying a white sub-pixel, not only does the white sub-pixel need to be turned on but also some of the R G B sub-pixels (in the same pixel) need to be turned on to compensate the color coordinate to the desired position. Moreover, in order to compensate the white sub-pixel, an additional driving function is required to control each R, G and B sub-pixel separately depending on the deviation of the white sub-pixel from the desired white point D65.
FIG. 3 is a block diagram illustrating a method of decoding an RGB format signal to an RGBW format signal for rendering images. Signals Ri, Gi and Bi 310 are input to a signal converter 320 converting the signals Ri, Gi and Bi 310 to output signals Ro, Go, Bo, Wo to RGBW format OLED device 350. Since the unfiltered white sub-pixel deviates from the desired white point, an additional driving function is required to compensate the white sub-pixel of the RGBW format OLED devices, thereby resulting in a more intricate image rendering method.
In an article entitled “High-Performance and Low-Power AMOLED Using White Emitter with Color-Filter Array” by K. Mameno et al., published by IDW in 2004, the authors disclose methods of decreasing the power consumption and reducing a color shift with a wide viewing angle.
U.S. Publication No. 2005/0040756, the entirety of which is hereby incorporated by reference, discloses an RGBW four color AMOLED including gamut sub-pixels that can substantially improve the efficiency and lifetime of the gamut sub-pixels.
For rendering images of an RGBW format OLED device, the algorithm of decoding an RGB signal to an RGBW signal is expressed in the following equations,(Ri,Gj,Bk)→(R′l,G′m,B′n,W′p)
where,
i, j, k, l, m, n, and p are the gray scalesp=min(i,j,k)×αl=i−p m=j−p n=k−p 
For power consumption reduction consideration the α is usually set as 1. And the luminance will be converted to for portion, i.e.,L(R′l)=L(Ri)−L(Rp),L(G′m)=L(Gj)−L(Gp),L(B′n)=L(Bk)−L(Bp), andL(W′p)
As mentioned above, the color coordinate of the white OLED must have some R, G, or B luminance added to compensate the white color coordinate to the desired position, therefore, it can be written as following equation,L(W′p)=L(wp)+L(rp)+L(gp)+L(bp)
where, L(wp) is the white OLED luminance, and L(rp), L(gp), L(bp) are the compensation luminance for red, green and blue sub-pixels, respectively.
In the actual pixel driving for the RGBW sub-pixels, the actual luminance for each sub-pixel to achieve the desired overall luminance will be,L(R′l)=L(Ri)−L(Rp)+L(rp),L(G′m)=L(Gj)−L(Gp)+L(gp),L(B′n)=L(Bk)−L(Bp)+L(bp), andL(W′p)+L(wp)
The actual data signal will be very complicated and when the color coordinate of the white OLED changes or improves, the L(rp), L(gp), and L(bp) compensation luminance will also change. To solve this issue, it may take more resources to modify the additional driving function, and also increase fabrication costs.